Conventionally, some kind of matting system is used to relieve standing stress and fatigue or to attenuate falling impact stress. Many such systems are known. Some use closed-cell foam or vulcanized rubber chips bound together to provide a shock. absorbing structure. Others use an array of deformable elastomeric cells under an elastomeric surface layer. The problems with most conventional matting systems are twofold. Foam and most other conventional impact attenuating structures actually get harder as the force applied to them increases, and they can bottom out with hip, elbow or head impacts, or even just prolonged standing. In most conventional systems, the force absorbing mechanism begins with an immediate displacement at the very surface of the mat, a displacement that often leads to dangerous foot entrapment and/or other displacement instability for those who play, walk or work on such a surface.
It is generally acknowledged that standing for long periods induces discomfort and fatigue, especially in the lower body. A high proportion (>80%) of industrial workers required to stand for long periods report lower leg or foot discomfort. It is also generally agreed and documented in several scientific studies that standing on a soft surface improves perceptions of comfort and reduces perceptions of fatigue or “tiredness”.
The results of research aimed at documenting the physiological basis for antifatigue mats are more equivocal. Standing for long periods often leads to edema (swelling) of the lower legs and feet and antifatigue mats can provide relief. The conventional view is that soft surfaces cause more adjustments to posture, activating the venous pumping that returns blood to the heart, thus reducing swelling and discomfort.
A recent study by Madeleine et al (1998), “Subjective, physiological and biomechanical responses to prolonged manual work performed standing on hard and soft surfaces”, Eur. J. Appl. Physiol. 77:1-9, however compares responses from people that work while standing for two hours on either hard surface or a soft antifatigue mat. The data reported in this study conflict somewhat with the conventional wisdom.
As to the need for impact safety matting, falls onto hard surfaces are a significant cause of injury and accidental death, especially among the elderly. Among non-fatal injuries due to falling, hip fractures are the most common and the most severe. The loss of mobility following a hip fracture is itself a potentially fatal risk and many elderly patients never return to normal activity after a fall. Given the significant human and medical cost of high hip fracture rates, researchers have explored ways of reducing the rate of hip fractures among the elderly. Proposed strategies include the use of protective hip pads, cushioned flooring and the promotion of exercise programs to increase the strength and agility of at-risk individuals. We review the injury reduction potential of cushioned floors, and calculations as to the effect of compliant flooring materials on the peak impact force acting at the hip.
The fracture strength of the femoral head has been estimated using mechanical tests of cadaveric specimens, finite element modeling and predictions based on material properties. From such studies, the peak lateral loads inducing fractures in older individuals range from 1000 to 6000 N. Younger subjects have greater femoral strength. Fracture strength depends on many factors, including the loading conditions and the age, body size and bone mineral density of the subject.
The force acting at the hip during a fall is affected by a number of factors, most notably the impact velocity, the effective mass involved in the impact, the material properties of the soft tissue overlying the hip and the properties of the surface against which the impact occurs. A group of researchers from Harvard University and Harvard Medical School (Robinovitch et al, 1991) used a constrained release experiment to determine the non-linear stiffness and damping properties of soft tissue and used their results to calculate the impact force on a hard surface. The predicted impact force magnitudes were similar to the breaking strength of the femoral neck, supporting the idea that unprotected falls onto hard surfaces can break the hip. The Harvard hip impact model uses non-linear stiffness and damping functions to describe the viscoelastic properties of the soft tissue overlying the hip and documents the soft tissue parameters for males and female subjects across a range of soft tissue thicknesses.
FIG. 22 is a schematic of the Harvard hip impact model, in which impact of the falling mass m is moderated by the compliant material properties of the soft tissue. Soft tissue behavior is characterized by non-linear stiffness (kt) and damping (ct) functions.
Equation of Motion
The response of the system in FIG. 22 is described by a differential equation in xm:F=m(d2xm/dt2)=ct(dxm/dt)=ktxm  (1)
In order to calculate the peak force of an impact, the model requires parameters for soft tissue properties, the effective mass and a description of the initial conditions (e.g. impact velocity) defining the impact.
Soft Tissue Properties:
Robinovitch (1991), gives non-linear functions for kt and ct for both males and females and for different muscle activation states. For the purposes of the analysis presented here, values for male subjects in a muscle-relaxed state were used. Specifically,kt=90,440(1−e−F/114)  (2)ct=756(1−e−F/108)  (3)Effective mass:Robinovitch (1991) reports the average effective mass (m) involved in hip impact to be 39 kg for males in the muscle-relaxed state.m=39.0  (4)Initial Conditions:
Van den Kroonenberg et al (1993) report estimated hip-floor impact velocities ranging from 2.14 to 4.25 m s−1 and averaging 3.19 m s−.dxm/dt(t=0)=3.19 ms−1  (5)Also at time t=0:    xm=0    xs=0    d2xm/dt2=g=9.81 ms−2 Example Solution
Equation 1 is integrated using the properties shown in Equations 2-4 and the initial conditions. FIG. 23 shows the force vs. time curve thus calculated. The peak force of 7022 N at 21.6 ms is similar to the value of 7120 N at 21.6 ms read from the graph in FIG. 6 of Robinovitch (1991). The 1.4% difference in peak force can be attributed to differences in the numerical integration techniques employed, or to errors in measuring peak force values from the graph.
Other conventional mat systems, and especially closed cell foam systems, typically provide dangerous surface deformation levels; this often leads to foot lock or foot entrapment in football games played on surfaces having a foam substructure. Closed cell foam also fails to adequately protect against injuries from serious or ‘bottoming-out’ impacts.
Conventional systems under football fields tend to compact with time, thus leading to both localized and generalized compaction and hardening of the energy absorption layer under the field. In health care centers and gymnasium-type sports floors, dense rubber has been used in addition to integrated foam pad packing. Dense forms of rubber do not provide bottoming-out protection and are relatively incompressible providing little protection from fall impacts.
Exercise mats made of foam and covered with thin layers of surfacing are easily torn through, thus creating a trip hazard, or they are too soft and can easily entrap the foot. Foam mats also do not provide adequate support activities that involve critical body part (head, hip, elbow and the like) or full body impact with the mat surface and can easily bottom out, allowing potentially injurious impact right through the mat and onto the harder floor surface beneath the mat.
As most conventional mats are compressed, the surface area around the foot is deformed and disturbed and creates a situation where the subject is working, walking or standing on an unstable surface.
Many mats in the market place are also flimsy. They are easily damaged by carts and vehicles that impact the edge of the mat. The replacement frequency is high for mats in these settings. Though the center of the mat is not worn out, the edge is damaged and therefore must be replaced. The mats also easily flip up or bunch up creating trip hazards.
What is needed is a mat system that exhibits little or no surface deformation at working loads, but which is resilient at levels below the surface to attenuate fatigue causing factors, and to safely absorb body impact in the event of falls or other body collisions with impact surfaces.